Apparatus for positioning an optical element in a structure

ABSTRACT

In an apparatus for positioning an optical element in a structure, particularly in an objective housing of a projection objective for microlithography, the optical element is connected to the structure via fastening elements. The position of the optical element is set by means of adjusting fasteners. The fastening elements are arranged in such a way and the adjusting fasteners can be actuated in such a way that the optical element can be tilted about three mutually independent axes and can additionally be displaced in a translatory fashion in one axial direction

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates to an apparatus for positioning an opticalelement in a structure, the optical element being connected to thestructure via fastening elements, and it being possible to set theposition of the optical element by means of adjusting elements. Theinvention relates, in particular, to a projection objective formicrolithography, a beam splitter cube being provided as opticalelement.

[0003] 2. Description of the Related Art

[0004] Manufacturing and assembly tolerances that lead to aberrations inthe objective inevitably occur during the assembly of optical imagingapparatuses, for example a projection objective for microlithography. Itis known for the purpose of compensating these tolerances to repositionone or more optical elements appropriately in the objective. The samealso holds for optical elements that need to be adjusted very exactly inthe beam path.

[0005] It is known from DE 199 01 295 A1, for example, to displace oneor more optical elements relative to the optical axis in order tocompensate manufacturing and assembly tolerances and to correctaberrations.

[0006] WO 99/66361 discloses a positioning device for a lens, threedegrees of freedom being present for positioning the lens.

SUMMARY OF THE INVENTION

[0007] It is an object of the present invention to provide an apparatusfor positioning an optical element in a structure, in particular anoptical element in an objective housing of a projection objective, thatpermits very exact positionings and adjustments in the beam path.

[0008] This object is achieved according to the invention by virtue ofthe fact that the fastening elements are arranged in such a way and theadjusting fasteners can be adjusted in such a way that the opticalelement can be tilted about three mutually independent axes (x-, y-,z-axis) and can additionally be displaced in a translatory fashion inone axial direction (z-axis).

[0009] The positioning apparatus according to the invention thereforehas a total of four degrees of freedom, as a result of which an opticalelement to be positioned and/or to be adjusted can be aligned veryprecisely in multifarious ways.

[0010] This holds, for example, inter alia for a beam splitter cube in aprojection objective. Whereas a light beam emanating from a lightsource, for example a laser, is deflected by the beam splitter cubeafter passing through a reticle, which represents the object, into acantilever arm of the objective housing, the light beam subsequentlyreturning from the cantilever arm is passed by the beam splitter cube inthe direction of the imaging plane, specifically a wafer.

[0011] Since, now, the beam splitter layer in the beam splitter surfacein the beam splitter cube functions as a deflecting mirror for the lightbeam coming from the reticle, the beam splitter cube must be capable ofbeing adjusted by two tilt axes that define the plane of the beamsplitter layer, in order to be able to compensate angular errors betweenthe optical axis in the beam path from reticle to beam splitter cube andthe optical axis in the beam path of the cantilever.

[0012] The external surfaces are to be as perpendicular as possible tothe optical axes of the beam path so that in this case the light beam atthe external surfaces, for example the top side, front side and the rearside, of the beam splitter cube is not too strongly deflected. In order,now, also to be able to adjust the external surfaces to the opticalaxes, there is a need for a fine adjustment about an axis of rotationand/or a tilt axis perpendicular to the plane of the beam splitter layerand/or to the beam splitter surface.

[0013] So that, now, the light beam impinging from the reticle on thebeam splitter cube is deflected exactly onto the optical axis of thebeam path in the cantilever, the plane of the beam splitter layer mustbe situated exactly at the point of intersection of the optical axes ofthe reticle beam path and cantilever beam path. In order to be able toposition the beam splitter layer exactly at the point of intersection,it must additionally be possible to finely adjust the beam splitter cubein a translatory fashion perpendicular to the plane of the beam splitterlayer.

[0014] According to the invention, the above named positionings andadjustments can be achieved with the aid of the inventive arrangementand configuration of the fastening elements and adjusting elements.

[0015] In a preferred refinement, it can be provided in this case that

[0016] a) the fastening elements in each case block the translationalong one axis, and all the axes of the translations blocked by thefastening elements lie in a plane that is defined by the tilt axes alongwhich the optical element cannot be displaced in a translatory fashion,

[0017] b) the axes of the translations blocked by the fastening elementslie perpendicular to the axial direction in which the optical elementcan be displaced in a translatory fashion, and

[0018] c) the axes of the translations blocked by the fastening elementsintersect one another at a point on an axis along which the opticalelement can be displaced in a translatory fashion.

[0019] In addition, it is possible in this case for the axes of thetranslations blocked by the fastening elements to intersect one anotherat a point on an axis along which the optical element can be displacedin a translatory fashion and which goes through the point ofintersection of the two tilt axes along which the optical element cannotbe displaced in a translatory fashion.

[0020] With a beam splitter cube as optical element, the beam splittersurface on the beam splitter cube is in this case the plane in which theaxes of the translations blocked by the fastening elementsadvantageously lie. The optical axis along which the translatorydisplacement is to be performed is in this case an axis that isperpendicular to the beam splitter surface. The origin of the coordinatesystem is likewise located in this case on the plane of the beamsplitter surface at the point of intersection of the optical axis of thereticle beam path with the optical axis of the cantilever beam path. Thethree tilt axes advantageously intersect one another in this case at theorigin of the coordinate system.

[0021] When the optical element is a mirror or a lens, the above namedplane in which the axes of the translations blocked by the fasteningelements lie advantageously contains the vertex of the surface of themirror or of the lens. One possibility for using the solution accordingto the invention would be, for example, an elliptic mirror or akidney-shaped mirror. The same holds for mirrors or lenses that areprovided with a corrective aspheric.

[0022] Advantageous developments and refinements of the invention emergefrom the remaining subclaims and from the exemplary embodiment describedbelow in principle with the aid of the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 shows an illustration of the principle with the mode ofoperation of a projection objective for microlithography;

[0024]FIG. 2 shows an enlarged perspective illustration of a beamsplitter cube provided with an adjusting and setting device and having aholding frame, from the side;

[0025]FIG. 3 shows the beam splitter cube according to FIG. 2 in aperspective illustration from above;

[0026]FIG. 4 shows an enlarged perspective illustration of a fasteningelement;

[0027]FIG. 5 shows an enlarged perspective illustration of an adjustingfastener;

[0028]FIG. 6 shows a perspective illustration with a translatorydisplacement;

[0029] FIGS. 7 to 9 show illustrations of various tilting possibilities;

[0030]FIG. 10 shows an embodiment with a bearing frame;

[0031]FIG. 11 shows an enlarged illustration of a fastening elementaccording to FIG. 10; and

[0032]FIG. 12 shows an enlarged illustration of an adjusting fasteneraccording to FIG. 10.

DETAILED DESCRIPTION

[0033]FIG. 1 illustrates the principle of a projection exposure machinehaving a projection objective 1 for microlithography for the purpose ofproducing semiconductor elements.

[0034] It has an illumination system 2 with a laser (not illustrated) aslight source. Located in the object plane of the projection exposuremachine is a reticle 3, whose structure is to be imaged on acorrespondingly reduced scale onto a wafer 4 that is arranged beneaththe projection objective 1 and is located in the image plane.

[0035] The projection objective 1 is provided with a first verticalobjective part 1 a and a second horizontal objective part 1 b. Locatedin the objective part 1 b are a plurality of lenses 5 and a concavemirror 6, which are arranged in an objective housing 7 of the objectivepart 1 b. A beam splitter cube 21 is provided for deflecting theprojection beam (see arrow) from the vertical objective part 1 a with avertical optical axis 8 into the horizontal objective part 1 b with ahorizontal optical axis 9.

[0036] After reflection of the beams at the concave mirror 6 andsubsequent passage through the beam splitter cube 21, these strike adeflecting mirror 10. At the deflecting mirror 10, the horizontal beampath 9 is reflected, in turn, into a vertical optical axis 11. A thirdvertical objective part 1 c with a further lens group 12 is locatedbeneath the deflecting mirror 10. Three λ/4 plates 13, 14 and 15 arefurther additionally located in the beam path. The λ/4 plate 13 islocated in the projection objective 1 between the reticle 3 and the beamsplitter cube 21 behind a lens or lens group 16. The λ/4 plate 14 islocated in the beam path of the horizontal objective part 1 b, and theλ/4 plate 15 is located in the third objective part 1 c. The three λ/4plates serve the purpose of completely rotating the polarization once,resulting, inter alia, in minimization of beam losses.

[0037] The individual optical axes of the projection objective 1 arecertainly adjusted very exactly to one another during construction ofthe objective such that they run with satisfactory accuracy parallel orperpendicular to one another, but owing to tolerance inaccuracies, orelse to faults that still occur during operation, it is not alwayspossible to achieve an alignment of the optical elements with theaccuracy required for the applications envisaged, and this leads to acorresponding worsening of the imaging quality.

[0038] Consequently, for the purpose of raising the imaging quality,measures need to be taken in order, in an appropriate way, exactly toposition and to adjust optical elements in the projection objective 1that are suitable therefor. Suitable, inter alia, for this purpose isthe beam splitter cube 21 which is provided to this end with anadjusting and setting device 17 that is described in more detail below.

[0039] The beam splitter cube 21 of FIG. 1 provided with the adjustingand setting device 17 can be seen in FIGS. 2 to 9 in an enlargedillustration with fastening elements 22 and adjusting fasteners 23. Thefastening elements 22 and the adjusting fasteners 23, which areconnected on one side to the beam splitter cube 21, make the connectionbetween the beam splitter cube 21 and a fixed holding frame 24 that isconnected (in a way not illustrated in more detail) to the objectivehousing of the projection objective 1.

[0040] The aim now is for it to be possible to tilt the beam splittercube 21 relative to the fixed holding frame 24 about three mutuallyindependent axes, and to be able to displace it in terms of onedirection in a translatory or linear fashion. For this purpose, theorigin 25 of the coordinate system x, y, z lies on the beam splittersurface or beam splitter plane 26. In this case, the x-axis 27 islocated parallel to the longitudinal axis of the beam splitter cube 21in the beam splitter plane 26, the y-axis 28 is located perpendicular tothe x-axis 27, likewise in the beam splitter plane 26, and the z-axis 29is located perpendicular to the beam splitter plane 26.

[0041] In order to be able to adjust the beam splitter cube 21, it mustbe possible to tilt it about the x-axis 27, the y-axis 28 and the z-axis29, and to displace it along the z-axis 29.

[0042] As may be seen, the beam splitter cube 21 is provided with twofastening elements 22 that are located in the corner regions of alongitudinal edge of the beam splitter cube 21. The two adjustingfasteners 23 are located in the corner regions of the longitudinal edgeof the beam splitter cube 21, which is situated opposite thelongitudinal edge with the two fastening elements 22. Of course, thearrangement of the fastening elements 22 and the adjusting elements 23is to be regarded only as an example. In particular, the adjustingelements 23 can also be provided at another location.

[0043] As may be seen from the enlarged illustration in FIG. 4, eachfastening element 22 has two translational mobilities 30 a and 30 b andthree rotational mobilities 31 a, 31 b and 31 c. The configuration ofthe fastening element 32 with a long rod 32 and a spherical joint 33 aand 33 b fitted in each case at the rod end fixes the third linearpossibility 30 c of displacement or translation, which runs in thedirection of the longitudinal axis of the long rod 32. The two sphericaljoints 33 aand 33 b result in an articulated connection or suspension ofthe beam splitter cube 21. Of course, the fastenings by means of thespherical joints 33 a and 33 bare to be regarded only as exemplary. Ifrequired, it is possible for this purpose also to provide solid jointssuch as, for example, devices of the leaf spring type having appropriateelasticity.

[0044] Since the beam splitter cube 21 is to be displaced along thez-axis 29, which represents the optical axis between the reticle 3 andthe beam splitter cube 21, the fastening elements 22 are arranged suchthat the direction of the fixed translation possibility 30 c is orientedperpendicular to the z-axis 29 (see FIG. 2).

[0045] In order to permit tilting about the x-axis 27 in the beamsplitter plane 26, the fixed translation possibilities 30 c of all thefastening elements 22 must lie in the plane that is defined by the xtilt axis 27 and the y tilt axis 28. Since the plane defined by thex-axis 27 and the y-axis 28 is identical in the case of the presentexemplary embodiment to the beam splitter plane 26, the fixedtranslation possibilities 30 c of the fastening elements 22 likewise liein the beam splitter plane 26.

[0046] In order to permit tilting about the z-axis 29, all the fixedtranslation possibilities 30 c of the fastening elements 22 must alsointersect the z-axis 29. It may be seen from FIG. 2 that the extensionsof the longitudinal axes of the long rods 32 of the fastening elements22 thereby meet or intersect one another at the point of intersection ofthe coordinate system 25, which is located in the plane of the beamsplitter plane 26.

[0047] When, as in the case of the exemplary embodiment illustrated, thebeam splitter cube 21 is moved and/or manipulated by two adjustingfasteners 23, the adjusting fasteners 23 have one translational mobility34 a and three rotational mobilities 35 a, 35 b, 35 c, while thetranslation possibilities 34 b and 34 c are held still via adjustingelements still to be discussed (and illustrated in FIG. 5, for exampleby the adjusting screws 40 a, 40 b, 40 c, 40 d) in the adjustingfasteners 23. By actuating the adjusting elements, the adjustingfasteners 23 can be adjusted in the translation directions 34 b and 34c, it thereby being possible to displace and tilt the beam splitter cube21 in the desired way.

[0048] An example of such an adjusting fastener 23 is furtherillustrated in FIG. 5. It has a spherical joint 36 that connects thebeam splitter cube 21 to a triangular plate 37. Located at the base ofthe triangular plate 37 is a hinge 38 with an appended sliding piece 39that is guided in one direction in the holding frame 24. With the aid ofadjusting elements in the form of adjusting screws 40 a, 40 b, 40 c, 40d, the sliding piece 39 can be displaced in a linear or translatoryfashion relative to the holding frame 24 in the mutually perpendiculartranslational mobilities 34 c and 34 b.

[0049] Instead of two adjusting fasteners 23 with in each case twoadjusting elements, it is also possible as an alternative to use fouradjusting fasteners with in each case one adjusting element, in thiscase each adjusting fastener then needing to have two translationalmobilities and three rotational mobilities, and it being possible forthe third translation mobility to be adjusted by an adjusting element(not illustrated).

[0050] Imagining away the fastening elements 22 in the system composedof beam splitter cube 21, fixed holding frame 24, adjusting fasteners 23and fastening elements 22, it is possible for the beam splitter cube 21to move in two degrees of freedom relative to the holding frame 24without actuating the adjusting elements in the adjusting fasteners 23.

[0051] The adjusting fasteners 23 must be arranged such that these twodegrees of freedom cannot coincide with the tiltings about the x-axis27, the y-axis 28 and the z-axis 29 and the translatory displacementalong the z-axis 29 or with a combination of these movements, so thatforces and torques that act in these directions of movement can besupported.

[0052] It can be seen from FIG. 6 how the beam splitter cube 21 can bedisplaced along the z-axis 29 when the adjusting fasteners 23 aredisplaced in the same sense in direction 34 b with the aid of theadjusting elements, specifically the adjusting screws 4(a and 40 c. Theadjusting screws 40 a and 40 c must be adjusted correspondingly in eachcase for this purpose.

[0053] If the two adjusting fasteners 23 are displaced in the oppositesense in direction 34 b (see arrows), this results in a tilt about they-axis 28 for the beam splitter cube 21, as may be seen from FIG. 7.

[0054] It can be seen from FIG. 8 that the beam splitter cube 21 istilted about the x-axis 27 when both adjusting fasteners 23 aredisplaced in the same sense in direction 34 c. The adjusting elements,specifically the adjusting screws 40 b and 40 d, are actuated asappropriate for this purpose.

[0055] In order to achieve a tilt of the beam splitter cube 21 about thez-axis 29, the two adjusting fasteners 23 must be displaced in theopposite sense in direction 34 c, as is illustrated by the arrows inFIG. 9.

[0056] The adjusting fasteners 23 or the adjusting elements can beadjusted by hand, by motor, pneumatically, hydraulically,electromagnetically, piezoelectrically, or magnetostrictively.

[0057]FIG. 10 shows an exemplary embodiment of the apparatus with abearing frame 41 and fastening elements 22 and adjusting fasteners 23,in the case of which the joints are designed as solid or spring joints.

[0058] The beam splitter cube 21 is mounted in the bearing frame 41,which is borne in the holding frame 24 by the fastening elements 22 andthe adjusting fasteners 23.

[0059] The beam splitter layer 26 can be seen as a line on the beamsplitter cube 21.

[0060] A fastening element 22 in accordance with FIG. 10 may be seen inan enlarged illustration in FIG. 11. It connects the bearing frame 41,in which the beam splitter cube 21 is mounted, to the holding frame 24.

[0061] By bending a leaf spring joint 42, the contact of the fasteningelement 22 with the bearing frame 41 has a translational mobility alongthe axis 30 a, and a rotational mobility about the axis 31 b.

[0062] By bending a leaf spring joint 43, the contact of the fasteningelement 22 has a translational mobility along the axis 30 b and arotational mobility about the axis 31 a.

[0063] Torsion of the leafspring joints 43 results in a rotationalmobility about the axis 31 c for the contact of the fastening element 22with the bearing frame 41, as a result of which the fastening elementwith the leaf spring joints 42 and 43 has the same mobilities as thefastening element assembled from the long rod 32 and the two sphericaljoints 33 a and 33 b (see FIG. 4). The fastening element 22 is stiff ina translatory fashion only along the axis 30 c.

[0064] An adjusting fastener 23 in accordance with FIG. 10 is shown inFIG. 12 with solid joints in an enlarged illustration. By bending a leafspring 44, the contact of the adjusting fastener 23 with the bearingframe 41 obtains a translational mobility along the axis 34 a and arotational mobility about the axis 35 c.

[0065] The contact of the adjusting fastener 23 with the bearing frame41 acquires a rotational mobility about the axis 35 b by torsion of theleaf spring 44.

[0066] In order to obtain a rotational mobility about the axis 35 a likethe adjusting fastener (see FIG. 5) formed from the spherical joint 36,the triangular plate 37 and the hinge 38, a block 45 adjoining the leafspring 44 (the component analogous to the sliding piece 39 of theexemplary embodiment already described) is mounted via leaf springs 46 aand 46 b on control levers 47 a and 47 b such that, with the controllevers 47 a and 47 b fixed, an instantaneous center of rotation with theaxis of rotation 35 a results at the point of intersection of theextensions of the leaf springs 46 a and 46 b.

[0067] One control lever 47 a is mounted via a leaf spring 48 a in thepart of the adjusting fastener 23 permanently connected to the holdingframe 24.

[0068] The control lever 47 a can be adjusted with the aid of theadjusting screws 40 a and 40 b, the leaf spring 46 a transmitting theadjustment onto the block 45 and thereby initiating a movement of thebearing frame 41 and the beam splitter cube 21.

[0069] One control lever 47 b is similarly mounted via a leaf spring 48b in the part of the adjusting fastener 23 permanently connected to theholding frame 24.

[0070] The control lever 47 b can be adjusted with the aid of theadjusting screws 40 c and 40 d (adjusting screw 40 d not being visiblesince it is covered. It presses with respect to the adjusting screw 40 conto the control lever 47 b), the leaf spring 46 b transmitting theadjustment to the block 45 and a movement of the bearing frame 41 andthe beam splitter cube 21 thereby being initiated.

[0071] Since, in the exemplary embodiment shown, the control levers 47 aand 47 b are rotated by 45° together with the leaf spring joints 46 a,46 b, 48 a, 48 b relative to the displacement directions 34 b and 34 c,it is necessary in each case for two control levers to be actuatedsimultaneously in order to obtain a pure displacement along thedirection of 34 b or 34 c.

[0072] The control levers 47 a and 47 b must be moved simultaneouslyinward or outward in order to displace the block 45.

[0073] One control lever must be moved inward and the other movedoutward for a pure displacement of the block 45 along the axis 34 c.

What is claimed is:
 1. An apparatus for positioning an optical elementin a structure, the optical element being connected to the structure viafastening elements, and it being possible to set the position of theoptical element by means of adjusting fasteners, wherein said fasteningelements are arranged in such a way and said adjusting fasteners can beadjusted in such a way that the optical element can be tilted aboutthree mutually independent axes and can additionally be displaced in atranslatory fashion in one axial direction.
 2. The apparatus as claimedin claim 1, wherein a) said fastening elements in each case block thetranslation along one axis, and all the axes of the translations blockedby said fastening elements lie in a plane that is defined by the tiltaxes along which the optical element cannot be displaced in atranslatory fashion, b) the axes of the translations blocked by saidfastening elements lie perpendicular to the axial direction in which theoptical element can be displaced in a translatory fashion, and c) theaxes of the translations blocked by said fastening elements intersectone another at a point on an axis along which the optical element can bedisplaced in a translatory fashion.
 3. The apparatus as claimed in claim2, wherein the axes of the translations blocked by said fasteningelements intersect one another at a point on an axis along which theoptical element can be displaced in a translatory fashion and which goesthrough the point of intersection of the two tilt axes along which theoptical element cannot be displaced in a translatory fashion.
 4. Theapparatus as claimed in claim 2, wherein the optical element is providedwith at least two of said fastening elements and at least two of saidadjusting fasteners.
 5. The apparatus as claimed in claim 4, wherein twoof said adjusting elements are provided for each said adjustingfastener.
 6. The apparatus as claimed in claim 1, wherein said fasteningelements and said adjusting fasteners are connected elastically to theoptical element.
 7. The apparatus as claimed in claim 6, wherein saidfastening elements are connected to the optical element via solidjoints.
 8. The apparatus as claimed in claim 6, wherein said adjustingfasteners are connected to the optical element via solid joints.
 9. Theapparatus as claimed in claim 1, wherein said fastening elements andsaid adjusting fasteners are connected elastically to a bearing frame inwhich the optical element is mounted.
 10. The apparatus as claimed inclaim 9, wherein said fastening elements are connected to said bearingframe in which the optical element is mounted via solid joints.
 11. Theapparatus as claimed in claim 9, wherein said adjusting fasteners areconnected to the bearing frame in which the optical element is mountedvia solid joints.
 12. The apparatus as claimed in claim 6, wherein saidadjusting fasteners are stiff in two axial directions and can beadjusted in a plane formed from the two stiff axial directions.
 13. Theapparatus as claimed in claim 12, wherein said adjusting fasteners areprovided with adjusting elements via which said adjusting fasteners canbe displaced in each case in the directions of the two stiff axes. 14.The apparatus as claimed in claim 13, wherein said adjusting elementscan be actuated independently of one another for the two stiff axes. 15.The apparatus as claimed in claim 1, wherein said optical element is abeam splitter cube.
 16. The apparatus as claimed in claim 2, whereinsaid optical element is a beam splitter cube.
 17. The apparatus asclaimed in claim 16, wherein the plane in which the axes of thetranslations blocked by said fastening elements lie is the plane of thebeam splitter surface of said beam splitter cube.
 18. The apparatus asclaimed in claim 15, wherein said fastening elements are arranged in thecorner regions of said beam splitter cube.
 19. The apparatus as claimedin claim 18, wherein said adjusting fasteners are arranged in the cornerregions lying opposite said fastening elements.
 20. The apparatus asclaimed in claim 1, wherein a mirror is provided as optical element. 21.The apparatus as claimed in claim 1, wherein a lens is provided asoptical element.
 22. The apparatus as claimed in claims 15, wherein theplane in which the axes of the translations blocked by said fasteningelements lie is a reference surface or optically used surface.
 23. Theapparatus as claimed in claim 1, wherein said structure is an objectivehousing of a projection objective for microlithography.
 24. A projectionobjective for microlithography, comprising a plurality of opticalelements arranged in an objective housing, at least one optical elementbeing connected to the objective housing via fastening elements, and itbeing possible to set the position of the optical element by means ofadjusting fasteners, wherein said fastening elements and said adjustingfasteners are arranged on the optical element, and can be actuated, insuch a way that the optical element can be tilted about three mutuallyindependent axes and can additionally be displaced in a translatoryfashion in one axial direction.
 25. The projection objective as claimedin claim 24, wherein a) said fastening elements in each case block thetranslation along one axis, and all the axes of the translations blockedby said fastening elements lie in a plane that is defined by the tiltaxes along which the optical element cannot be displaced in atranslatory fashion, b) the axes of the translations blocked by saidfastening elements lie perpendicular to the axial direction in which theoptical element can be displaced in a translatory fashion, and c) theaxes of the translations blocked by said fastening elements intersectone another at a point on an axis along which the optical element can bedisplaced in a translatory fashion.
 26. The projection objective asclaimed in claim 25, wherein the axes of the translations blocked bysaid fastening elements intersect one another at a point on an axisalong which the optical element can be displaced in a translatoryfashion and which goes through the point of intersection of the two tiltaxes along which the optical element cannot be displaced in atranslatory fashion.
 27. The projection objective as claimed in claim24, wherein a beam splitter cube is provided as optical element.